Well, after two and a half days of using my Fairphone 3 again as my everyday phone, I can confirm that the screen has not shown any more problems since the cleaning of the contacts. I hope it will last for many years 
, so I’m marking the topic as solved.
Unfortunately, the charging problem is still present: it doesn’t always detect the charger correctly, despite having cleaned the contacts of the module and the USB-C itself a few months ago 
.